How Can I Fix This Gap In My Fluid Domain?
Increase the resolution to reduce the gap.

The "glitch" you are experiencing is indeed an artifact of the low-resolution simulation.

The fluid simulation is computed on a grid of N × N × N small cubes, N being your "Final" resolution setting. Every detail of the 3D world, like the boundary location or an obstacle, that is smaller than the diameter of these cubes may be ignored by the simulation. Therefore, you should always expect gaps that are as large as one N-th of the edge of the simulation box.

In your example, you are simulating at N = 65 and visualizing the "Preview" mode at N = 45. Since your gap is around 1/45-th of the box size, it shouldn't be considered a glitch.

Increase your "Final" simulation resolution to ~150 and switch the "Viewport display" to "Final" to see the difference it makes.



Your file simualted at Final = 300 and Viewport display = Final